Measurements of nitrous acid and nitrogen oxides in the Church of San Luigi dei Francesi, Rome, Italy: relationship with the concentration outside.

Simultaneous indoor and outdoor samples collected in the Church of San Luigi dei Francesi were analyzed for nitrous acid and nitrogen oxides in the gas phase. Indoor levels of nitrogen oxide and nitrous acid were found to largely exceed those outdoors, while for nitrogen dioxide lower indoor levels were measured. These results show that indoor production of nitrous acid could result from heterogeneous reaction indoors on thè walls and exposed surfaces and that a non-reactive species such as nitrogen oxide penetrates indoors where it remains segregated and is very slowly depleted due to its small deposition velocity.